Mechanical Properties of Human Tooth Enamel: Patient Age and Spatial Distribution
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In this study the influence of aging on the mechanical behavior of human enamel was evaluated using 3rd molars from young (18-30 years) and old (55) patients. The elastic modulus and hardness were quantified using nanoindentation as a function of distance from the Dentin Enamel Junction (DEJ) and within three different regions of the crown (i.e. cervical, cuspal and inter-cuspal enamel). The apparent fracture toughness of human enamel was estimated using the indentation fracture resistance test. The hardness, elastic modulus and apparent fracture toughness were then used in estimating the brittleness according to a model that accounts for the competing dissipative processes of deformation and fracture. The brittleness of human enamel was compared to that of selected dental restorative materials (i.e. porcelain, ceramic and micaceous glass ceramic (MGC)) that are used for crown replacement. Results of the evaluation showed that the elastic modulus and hardness increased with distance from the DEJ in the all three regions examined, regardless of patient age. The largest increases with distance from the DEJ occurred within the cervical region of the old enamel. Overall, the results showed that there were no age-dependent differences in the properties of enamel near the DEJ. However, near the tooth's surface both the hardness (0.025) and elastic modulus (0.0001) were significantly greater in the old enamel. At the surface of the tooth, the average elastic modulus of old enamel was nearly 20% greater than that of the enamel from young patients. The apparent fracture toughness of the young and old enamel ranged from 0.74 to 0.92 MPa*m0.5 and from 0.67 to 0.88 MPa*m0.5, respectively. The old enamel had significantly lower toughness than the young enamel at the outer surface. The average brittleness of the young and old enamel increased with distance from the DEJ. For the old enamel the average brittleness increased from approximately 300 µm-1 at the DEJ to nearly 900 µm-1 at the occlusal surface. While there was no significant difference between the two age groups at the DEJ, the brittleness of the old enamel was significantly greater (and up to four times higher) than that of the young enamel near the occlusal surface. The brittleness numbers for the restorative materials were up to 90% lower than that of the old enamel at the occlusal surface. Based on results of this study, it was concluded that the brittleness index could serve as a useful scale in the design of materials used for crown replacement, as well as a quantitative tool for characterizing degradation in the mechanical behavior of enamel.